Determination of Threshold Electric Field of Practical Earthing Systems by FEM and Experimental Work

In this paper, the results of an experimental study and finite-element method (FEM) of the impulse characteristics of simply built practical earthing systems consisting of 2, 3, and 4 rods are reported. These electrodes are installed at three outdoor test sites having nonuniform soil and different soil profiles to provide variation in the result analysis. The tests on these electrodes were intended to determine the nonlinear characteristics of earthing systems which can be seen from their voltage and current traces and reduction of their impulse resistance values. Two-rod electrodes in the highest soil resistivity profile showed the largest reductions in impulse resistance due to soil ionization compared with other earth electrodes. Computer simulations of all the nine earthing systems using the FEM are also conducted, which showed good agreement with the measured results. It was also found from FEM that threshold electric field Ec is dependent on the earthing systems configurations, soil profile, and impulse resistances.